1. Field of the Invention
The present invention relates to a control device for an automatic transmission of a vehicle, and particularly, the present invention relates to a control device for an automatic transmission of a vehicle capable of generating automatic down shift in a manual shift mode at appropriate timing.
2. Description of the Related Art
An automatic transmission for a vehicle is provided with a plurality of ranges (shift positions such as Parking (P), Reverse (R), Neutral (N), Drive (D), Second (2) and Low (1)). The Drive range that is an automatic gear shifting range functions as an automatic shift mode in which automatic gear shifting is carried out on the basis of vehicle speed indicating a driving state and a throttle valve opening degree indicating an engine load in accordance with a gear shifting characteristic (shift map) stored in advance. Further, there is an automatic transmission configured so as to function as a manual shift mode (driver instruction mode) in which switching of an operation mode from the automatic shift mode allows a driver (or operator) to instruct shift up or shift down manually via a shift lever or a switch provided on a steering.
In the manual shift mode, driveability may be lowered because of reasons as follows. Namely, in setting of a gear (gear ratio) required by a driver, acceleration feel is lacking due to lowering of the number of revolutions of an engine, or engine braking hardly works by selecting a high gear ratio at low vehicle speed. In order to solve these problems, an automatic down shifting function is provided in the manual shift mode. The automatic down shifting function carries out down shift independently of a manual operation of a driver if needed. In the automatic down shifting function of the manual shift mode, distribution of vehicle speed for carrying out automatic down shift in each gear (that is, a shift map) is set up in advance. A conventional control device of an automatic transmission shown in Japanese Patent No. 3638389 is adapted to carry out control to switch vehicle speed distribution for automatic down shift between a flat road side and an uphill road side on the basis of estimated uphill gradient of the vehicle.
On the other hand, a torque converter is mounted on the automatic transmission. A lock-up clutch is provided in most of the torque converters. Engagement of the lock-up clutch causes a power transmission efficiency to be heightened, whereby fuel economy of the vehicle is improved. However, in order to further improve fuel economy, it is desired to broaden an engagement region of the lock-up clutch. However, since variation in engine torque is directly transmitted to a drive system in the engagement region of the lock-up clutch, broadening of the engagement region causes increase in noise or vibration.
Thus, technique to suppress transmission of the variation in engine torque to the drive system by carrying out slip control to cause the lock-up clutch to slip slightly is used. However, when the lock-up clutch is caused to continuously slip, a calorific value of a friction member (facing member) with which the lock-up clutch is provided increases to become high temperature. In the case where such a state of high temperature continues for a long time, a mirrored process, abrasion, carbonization or the like of a surface of the friction member occurs, there is a fear that a decrease of frictional force occurs. Further, when the friction member becomes high temperature, an increase in a slippage amount of the torque converter causes heat generation by a work loss to increase in addition to heat generation of the friction member.
In the automatic down shifting control of the conventional manual shift mode, switching of vehicle speed tables (vehicle speed distribution) for the automatic down shift is carried out on the basis of driving gradient of the vehicle. In addition, two kinds of vehicle speed tables for a flat road and an uphill road are merely switched. However, a driving force margin affecting a traveling performance of the vehicle is determined depending upon many factors in addition to the driving gradient. Thus, there has been a problem that automatic down shift is not always generated at appropriate timing in the case where switching of vehicle speed tables is carried out on the basis of only the driving gradient. For example, in the case of a cruise driving state on an uphill road (traction state on the uphill road and the like), the driving force margin of the vehicle becomes smaller, vehicle speed does not increase smoothly and tends to stagnate. However, automatic down shift may not be generated even in such a situation.
In this way, there has been a fear that vehicle speed stagnates in a region in which a slip ratio of the lock-up clutch is large (loose region) for a long time if a state in which the driving force margin is small continues and the vehicle speed does not increase smoothly. There has been a problem that this causes a facing calorific value of the lock-up clutch to increase and temperature of hydraulic oil (ATF) in the torque converter is thereby raised.
Further, there has been a problem that automatic down shift is generated even in a situation that down shift is not required essentially, for example, the case where the driving force margin is a medium degree or more and the vehicle speed increases smoothly when a vehicle speed table for an uphill road is selected in automatic down shift control. In this regard, since a wide variety of shift maps are prepared in accordance with estimated uphill gradient in the automatic shift mode (D range), a region in which the driving force margin is large is appropriately selected in accordance with a driving state of the vehicle. Therefore, a problem that the slip ratio of the lock-up clutch stagnates in the loose region for a long time hardly occurs.
Moreover, in the automatic down shifting control of the conventional manual shift mode, vehicle speed at which automatic down shift is generated has given distribution with respect to throttle valve opening degrees indicating an engine load. For that reason, in the case where the vehicle drives on a light uphill road or a flat road at the same vehicle speed when automatic down shift vehicle speed suitable for a cruise driving state on an uphill road (state in which the throttle valve opening degree is high) is set up, automatic down shift is generated in a state in which the throttle valve opening degree is very low or zero. There has been a problem that this causes driving force during deceleration (engine brake) of the vehicle to overwork to deteriorate driveability of the vehicle.